> How can I run two or more instances of the Firebird 2.1 on Windows?

The instsvc tool of the Windows 2.1.1 server has a new switch -name
that you can use to distinguish the various instances.

So you can copy the files from the Firebird 2.1 Zip redistributable to
the folder for the Firebird server and then install the service with

instsvc install -name "MyInstanceName"

and start your instance with

instsvc start -name "MyInstanceName"

You shouldn't register a client (instreg) on such a machine. The tools
(like GBAK, ISQL, GSEC) will find their libraries anyway.

HTH

Best Regards

Stefan